Photoconductive materials are extensively used in electrophotography and the like. For example, it is conventional to electrostatically charge a surface of such photoconductive material and then expose the charged surface to a light pattern so as to selectively discharge the areas of the surface impinged upon by the light pattern. The electrostatic pattern remaining on the photoconductive surface forms a latent image which can be sensed electronically or made visible by contact with a suitable developer such as charged toner particles or the like. The developed image on the photoconductive surface can be used as the finished copy or can be transferred (printed off or the like) to one or more copy sheets. Commercial usage of such processes is growing and there is a continuing need for faster, less expensive components capable of producing high resolution copies.
Various types of photoconductive materials have been used for such purposes, including sulfur, selenium, zinc sulfide, antimony oxide, cadmium sulfide, lead sulfide, antracene, anthraquinone and other organics, in the form of coated sheets, plates, drums, etc. One of the most widely used of such photoconductive materials is white zinc oxide in particulate form dispersed in a binder so as to provide a photoconductive coating on a suitable conductive substrate such as paper or the like. The binder usually is an insulating medium. A surface coating of such zinc oxide particles in a binder has many advantages, including the fact that it is relatively inexpensive and easily made and can be used, together with the underlying substrate, as the finished copy sheet.
Prior art of interest includes U.S. Pat. Nos. 2,710,813, 2,971,859, 3,121,007, 3,152,894, 3,178,312, 3,346,381, 3,347,702, 3,428,452, 3,466,183, 3,467,497, 3,573,906, 3,607,363, 3,634,134, 3,634,333, 3,636,492, 3,658,523 and 3,647,430 and Canadian Pat. Nos. 639,318 and 678,917.
Zinc oxide-containing photoconductive coatings have some drawbacks. For example, zinc oxide is most sensitive to light in the ultraviolet region, with a peak around 385 nm. However, it is desirable for many purposes to utilize visible light in connection with the production of the latent electrostatic image in the photoreproduction process. Accordingly, various types of organic sensitizing dyes have been added to zinc oxide to shift (and broaden) its spectral response into the visible light spectrum. Although these dyes generally initially perform satisfactorily, upon repeated exposure, the dye tends to undergo irreversable photo-destruction, resulting in a decrease in concentration and shift in spectral response, decreasing in visible light sensitivity while increasing in ultraviolet sensitivity. Since a fixed light source is used in the copying process, the visible images produced will show changes in density and contrast from copy to copy over a period of time as the spectral response shifts.
Accordingly, there has been a need for a relatively inexpensive photoconductive layer which would have all the advantages of organic dye sensitized zinc oxide but which would, in addition, be stable against spectral response shift upon repeated usage of such a layer. It would also be desirable if the light sensitivity of such a layer could be increased. It would further be desirable to render the coating bi-chargeable, that is, capable of being either negatively charged or positively charged, depending on the desired application, type of toner to be employed and other factors.
The foregoing needs have now been satisfied by the improved photoconductive layer embodiments of the present invention and by the method herein of providing photoconductive particles. In accordance with the present invention, a relatively minor amount of a sensitizer compound comprising one or more of certain Group IIB-VIA compounds is combined with zinc oxide and greatly increases the spectral response of zinc oxide so as to make it panchromatic. Moreover, this response is not subject to material degradation during reuse of the coating even over a great many exposures of the photoconductive layer. The Group IIB-VIA sensitizer compounds suitable for use herein are those compounds in which the VIA moiety has an energy band of less than 2.1 electron volts; i.e., selenium and tellurium. Thus, the additive compounds are zinc selenide, zinc telluride, cadmium selenide, cadmium telluride, mercuric selenide and mercuric telluride. The cadmium compounds are preferred and particularly cadmium selenide.
The present method of obtaining the improved photoconductive particles for use in the layer is simple and the layer is very inexpensive to make and is easily reproduced. The sensitizer compound can be provided in the photoconductive layer by merely physically mixing it in particulate form with the zinc oxide particles. However, when the sensitizer compound is chemically deposited as a coating on the surface of each of the zinc oxide particles, further advantages are obtained. In this regard, the resulting particles are bi-chargeable with approximately equal response in the positive and negative modes. Moreover, such particles exhibit greatly increased light sensitivity while still providing high resistance to spectral response shift. Further advantages of the present invention are set forth in the following detailed description and accompanying drawings.